The mammalian hippocampus is known to be critical for the formation of long-term memories, yet this brain region is highly vulnerable to epilepsy. Long-term potentiation and depression (LTP and LTD) - two forms of "Hebbian" synaptic plasticity- are widely regarded as likely cellular mechanisms of information storage in hip- pocampal circuits. A different form of synaptic plasticity at hippocampal synapses - homeostatic synaptic plasticity -drives compensatory changes at synapses to stabilize network function when overall circuit activity changes. Since Hebbian forms of synaptic plasticity are long-lasting, how these changes in synaptic efficacy endure in the face of homeostatic mechanisms that would be predicted to reverse them is unknown. Theories have been proposed regarding how these ostensibly conflicting plasticity processes could be interacting but experimental support for these theories is scarce, largely because the conventional preparations and time- course over which homeostatic plasticity is often studied differ from those most widely used (acute hippocampal slices) to study Hebbian plasticity. To address this issue empirically, our laboratory has characterized a rapid form of homeostatic plasticity at CA3-CA1 synapses in acute hippocampal slices, and my preliminary data reveals that one form of Hebbian plasticity (LTD) constrains such homeostatic compensation in an input- specific fashion. Given that recent work has linked homeostatic overcompensation with the development of epileptoform activity in hippocampal circuits, alterations in this inhibitory regulation of homeostatic plasticity may play an important role in the pathogenesis of temporal lobe epilepsy. This proposal will now test the hypothesis that local protein synthesis in dendrites, in addition to allowing for long-lasting information storage, plays a novel role in allowing Hebbian plasticity to constrain local homeostatic compensation at hippocampal synapses. This hypothesis will be tested in two specific aims. The objective of aim #1 is to examine how Hebbian plasticity interacts with homeostatic plasticity at the same synaptic inputs. I will ex- amine whether this interaction reflects an inhibition of the homeostatic activity sensor that detects changes in activity or reflects modulation of the compensation process directly. The goal of aim #2 is to determine whether local dendritic protein synthesis mediates the ability of Hebbian plasticity to constrain homeostatic plasticity at the same synaptic inputs. This proposed research is significant and innovative because it provides the first experimental approach to define how homeostatic and Hebbian processes influence one another in a defined neural circuit prone to epileptogenesis. PUBLIC HEALTH RELEVANCE: Alterations in synaptic connections between neurons in the hippocampus are thought to contribute to learning and memory, yet this brain region is also highly susceptible to epilepsy. More recent work has identified a novel class of synaptic modification - termed homeostatic synaptic plasticity - that is thought to stabilize activity within neural networks, but how this form of synaptic plasticity interacts with modifications important for learning is not known. The proposed work will examine how homeostatic forms of synaptic plasticity interact with synaptic modifications important for learning and memory, and will thus critically inform future studies that target homeostatic plasticity as a novel therapeutic option for epilepsy with the potential for permanently restoring stable patterns of activity in seizure-prone circuits.